Types of Down Syndrome

The most common of the three types of Down Syndrome is often referred to as Trisomy 21. About 95% of people with Down Syndrome have this. This type of Down Syndrome occurs because of an error in cell division. The error happens either before or at the time of conception. What happens is that a pair of the 21st chromosomes in either the egg or the sperm do not separate properly. This extra chromosome is then found in every cell in the body,thereby causing the characteristics of Down Syndrome.

Mosaicism occurs in about 1-2% of all people with Down Syndrome. In this type of Down Syndrome the error in separation of the 21st chromosome occurs in one of the first few cell divisions AFTER fertilization. This causes the fetus to have some cells with 46 chromosomes and some with 47. The cells with 47 chromosomes have one extra 21st chromosome. Because of the fact that not all cells contain the extra chromosome 21 the range of physical problems varies depending on the ratio of cells with 46 chromosomes to those with 47 chromosomes.

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Translocation occurs in approximately 3-4% of babies born with Down Syndrome. In this type of Down Syndrome a part of chromosome number 21 breaks off and attaches itself to another chromosome. Often this is chromosome number 14. This causes all cells in the body to have the extra piece of the 21st chromosome. When a child is born with this type of Down Syndrome it could mean that one of the parents is carrying chromosomal material that is unusually arranged.

The test used to determine which type of Down Syndrome is present in an individual is called karyotyping.